As a first-order feedback control power supply system of a power-supply device feeds back a part of the output of a CR smoothing filter provided in parallel with an LC output filter to an error amplifier for control, proper output voltage acquired in the LC output filter is outside a control loop. Therefore, a phenomenon that output voltage drops by an effect upon load current of the equivalent series resistance (ESR) of an inductor L of the LC output filter occurs. The phenomenon becomes remarkable as output voltage decreases, voltage accuracy in required output specifications is deteriorated in low output such as the output of 1 V and a problem that required specifications cannot be achieved occurs.
Besides, though a power supply control system, a circuit block and a purpose are different, there is an example in which a CR filter is provided to both ends of an inductor L for a detector of over-current and current (for example, a patent document 1). In this case, it is a condition that relation of L/ESR=C·R is met.
Patent document 1: JP-A 2000-227808